I. Field of the Invention
The present invention relates to a method for forming a surface configuration having a flat surface on a semiconductor substrate and, more particularly, to a method for forming a flat film on a surface configuration having a stepped or irregular surface, thereby allowing an accurate micro-patterning of the surface configuration.
II. Description of the Prior Art
Many micro-patterning processes have recently been developed to perform higher function and higher integration in a field of fabricating an ultrafine semiconductor device such as a semiconductor integrated circuit. In the manufacture of a semiconductor device, insulating layers and electrode wiring layers are formed on a semiconductor substrate. Thus, stepped or irregular surface is frequently formed on the surface configuration of the substrate, making the micro-patterning of the surface configuration of the substrate difficult. In other words, it is, for example, difficult to uniformly form a resist film on such uneven surface configuration, and the influence of the uneven surface results in remarkable decrease in the resolution of the resist pattern. In order to solve this problem, a multilayer resist system has been developed. For instance, such a multilayer resist system was described in Solid State Technology/August 1981 and J.Vac. Sci. tech 16(6) Nov./Dec., 1979. In this multilayer resist system, at least one layer of film is formed between a resist film and a substrate. In many cases, this film is a sufficiently thick resist film which planarizes the uneven surface. The thick resist film provides a flat supporting surface for the resist film of the uppermost layer, and prevents the resolution from decreasing due to random reflection of a light from the uneven surface in a photolithography, and supplements insufficient dry etching resistance of the resist in an electron beam, X-ray or deep UV lithography.
Specifically, the above-described planarizing film includes, depending on an etching method to be used, a positive resist baked at 200.degree. C. for 1 hour, a polymethylmethacrylate baked at 170.degree. C. for 30 min., and a deep UV resist film softly baked suitably to be executed with simultaneous transfer exposure by the deep UV. However, as a result of the present inventors' studies on multilayer resist system of this type, the following problems have arisen.
The surface of a baked positive photoresist which has heretofore been used can considerately planarize the uneven surface, but yet reflects the uneven surface as compared with the surface appearing after coating and before baking, and the surface is not safficiently flattened. The resist film often decreases in thickness at baking, and the surface unevenness of the film increases as compared with the surface immediately after coating. This is because, firstly, the positive photoresist shrinks in volume at curing by baking, and, secondly, the positive photoresist completely cures before being flattened, though the photoresist is fluidized by heat. Further, the positive resist is partially decomposed by baking, which may be another cause of decreasing the thickness of the film.
Thus, the uppermost resist film is not uniform in thickness even if it is formed through the above-described planarizing film, and its resolution is consequently decreased, thereby lowering the transfer accuracy of a predetermined pattern. In order to establish substantially completely flattened surface, it is known to increase the thickness of the planarizing film, but this results in a remarkable decrease in the transfer accuracy of the pattern.